1. Field of the Invention
The present invention relates generally lasers, and more specifically to a distributed feedback laser suitable for reducing noise caused by reflecting light.
2. Description of the Related Art
Semiconductor lasers have been extensively employed as light sources of optical fiber links, and connectors have been used for coupling the lasers with the fiber links. Some of the output energy of a semiconductor laser is often reflected off the connector or other optical components and returns to the laser. The returning light enters the laser and causes noise inside the laser. Such noise can lead to an error in transmitted symbols. Optical isolators have been developed to overcome this problem. However, they are not suitable for applications where cost savings is of primary importance.
Japanese Laid-Open Patent Specification Hei 4-17384 discloses a distributed feedback laser where the active layer is divided into a current injection region and non-injection region adjacent the end face of the laser through which the laser output is emanated. The diffraction grating structure of the non-injection (non-excited) region is used as a distributed reflector to prevent reflecting light from entering the laser through the output end face. However, the non-excited region has the same reflectivity to the output beam as well as to returning light rays. As a result, it is difficult to reduce the effect caused by the returning light rays. Additionally, because of the high loss of the non-excited region, difficulty is encountered in implementing low threshold lasers.